1. Field of the Invention
The present invention relates to a mechanism for moving a tape to be printed through a printing apparatus.
2. Description of the Prior Art
Printing apparatus of the type which prints characters on a tape which moves through the printing apparatus are well known in the art. The tape and a color carrier, such as a ribbon, pass through the printing station of the apparatus in printing registration with a selected raised character on a printing font. The font is mounted to the apparatus for rotation, and the particular character to be printed on the tape is placed at the printing station by rotating the font. Suitable pressure means forces the tape and the color carrier against the raised character on the font causing transfer of an image of the selected character to the tape. U.S. Pat. No. 3,834,507 discloses this particular type of printing apparatus.
In high quality printing apparatus of the type with which the present invention is intended to be used, the width of any given character differs from that of other characters. That fact can be appreciated by comparing, for example, the letter I with the letter W. Therefore, the apparatus should have means for advancing the tape through the apparatus different amounts between printing strokes, depending on the particular characters printed at each stroke, to improve the appearance of the printed tape.
The appearance of the printed tape can be improved even more if the apparatus has means for back-spacing the tape. Certain letters, when placed adjacent to each other, produce an optical illusion of being spaced a greater distance than other adjacent letters, even when that distance is in fact the same. In the art, this is known as "letter interlock". Take, on one hand, a situation in which the letters H and M are adjacent and spaced a given distance from each other and, on the other hand, one in which the letters A and W are adjacent and spaced that same distance from each other. The A and W will appear to be farther apart than the H and M because of the particular slope of their adjacent edges. With a back-space means, the operator can compensate for that illusion. For instance, assume that the advancing means produces the same spacing between all letters. An A has just been printed on the tape. If an H follows the A, then that spacing will be correct. However, if a W follows the A, then that spacing will appear excessive because of letter interlock. To compensate for this, the operator uses the back-space means before the W is printed, causing the W to be printed closer to the A.
The means for advancing and back-spacing the tape play a critical role in achieving the goal of proper letter spacing in the kind of printing apparatus which has been described. The final appearance of the printed tape depends heavily on how well the tape is positioned between printing strokes by the mechanism or mechanisms for advancing and back-spacing the tape.